Apparatus for cooling and calcining



Oct. 28, v1958 H. NIEMS 2,858,123

APPARATUS FOR COOLING AND CALCINING Filed Feb. 9, 1955 2 Sheets-Sheet 1F 1g. 'i E g. 2

@MM M 5L United States Patent APPARATUS FORCOOLING AND CALCINING LeeI-I. Niems, Homewood, Ill., assignor tn Marblehead Lime Company,Chicago, Ill., a corporation of Delaware Application February 9, 1955,Serial No. 487,117

3 Claims. (Cl. 263-32) The present invention is directed to an improvedapparatus for the heat treatment and cooling of calcareous material, and`in particular relates to an improved apparatus for calcining andcooling lime particles.

Naturally occurring limestone rocks are frequently calcined to convertthe calcium carbonate in the limestone to Substantially pure calciumoxide in a rotary, inclined kiln with the use ot' a direct ame as thesource of heat. When the kiln is rotated, however, the smaller sizestend to accumulate towards the center of the load in the kiln Where theyare not subject to direct radiation from the flame, nor are they incontact with hot lining of the kiln.` Because of their location, thesesmall pieces at the center frequently are not completely calcined, sothat they retain a limestone core.

Attempts to make the calcination treatment more thorough by keeping theparticles in the furnace for a longer time, or by increasing thetemperaturev do not always func'- tion satisfactorily because increasingthe severity of the treatment frequently results in overburning alreadycalcined particles, causing a discoloration and other eiects which arecompletely undesirable in the product, as well as an increase in thefuel requirements.

. The present invention is directed to an apparatus for completing thecalcining of lime particles as they leave the rotary kiln, followed bycooling the particles in an etiicient manner to a temperature at whichthe particles can be handled The chemical process involved in thecalcination of calcium carbonate is represented by the followingequation:

In accordance with the present invention, the concentration of carbondioxide produced in completing the calcining reaction is continuouslyremoved, at least in part, so that the calcining reaction proceeds morenearly to completion.

in the apparatus of the present invention a soaking zone is combinedwith a cooling zone in such a manner that the air iiow through thecooling zone is used more efficiently. Specifically, in the apparatus ofthe present invention, the material leaving the rotary kiln dropsdirectly into a soaking zone where any incompletely calcinated limeparticles remain until such time as the calcination is completed by thetransfer of the sensible heat from the lime particles themselves. Thecalcined particles are discharged as a plurality of separate streamsinto a cooling zone where the particles are contacted with acountercurrent stream or streams of air.

In order to avoid decreasing the temperature in the soaking zone belowthat required for eiiicient calcination, most of the air leaving thecooling zone is diverted around the soaking zone but recovered, for itssensible heat, and put to other uses. However, a portion of the airbeing passed through the cooling Zone is directed upwardly through thesoaking zone so as to purge the soak- 2,858,123 Patented Oct. 28, 1958ICC Experiments have indicated that where the carbon dioxide content ofthe soaking zone atmosphere is maintained at a level lower than thatwhich would naturally occur without purging, calcination of theparticles takes place at lower temperatures. Consequently, a greateraverage temperature differential exists between the hot lime particlesand the uncalcined core of the particles. Since there is a greatertemperature differential, a greater amount of heat transfer to the corewill take place, and at a greater rate, thereby completing the calciningmore rapidly.

An object of the present invention is to provide an improved apparatusfor calcining of calcareous material, particularly limestone.

Another object of the invention is to provide an improved apparatusincluding a combined soaking and cooling zone which cooperate to achieveefficient calcining as well as efficient heat transfer in the coolingzone.

Another object of the invention is to provide an irnproved apparatus forby-passing cooling air abo-ut a soaking zone, while permitting a smallfraction of the cooling air to serve the function of purging `thecooling zone of some of its carbon dioxide content.

These and other objects and features of the invention will be moreapparent to those skilled in the art from the following description ofthe attached sheets of drawings which illustrate preferred embodimentsof the invention.

In the drawings:

Figure l is a front elevational View of the apparatus which can beemployed in the present invention;

Figure 2 is a side elevational view of the apparatus;

Figure 3 is a cross-sectional plan View of the embodiment of theinvention illustrated in the preceding two figures; and

Figure 4 is a cross-sectional plan view of a modified form of theinvention.

As shown in the drawings:

Figure 1, reference numeral 10 indicates generally a rotary kiln of thetype employed to calcine limestone particles, the kiln 10 receiving oneor more burners at the discharge end. The burners l1 yare positioned todirect streams of ilame from the combustion of powdered coal, oil, orthe like into the kiln 10.

The kiln 10 is suitably inclined so that the calcined and partiallycalcined particles leaving the kiln 10 fall by gravity onto a grate 12having perforations 12a which permit all the lime particles to passthrough, but retain any large pieces of ash ring or coatingwhich maypass from the kiln.

As best seen in Figure 2, the grate 12. is inclined to the horizontal sothat the larger particles accumulating on the grate may roll off bygravity or may be raked off into a discharge chute 13 from which thelarger particles may be removed periodically by opening an access door14 provided for that purpose. Particles of lime pass through the holes12a and pass into a soaking zone generally indicated at numeral 16 inthe drawings. In the zone 16, the particles are unheated, butcalcination of any uncalcined fraction is continued by the heatcontained in the sensible heat of the particles.

In passing the calcined particles from the soaking zone to the coolingzone, it is desirable to avoid as much as possible the formation of acentral cone of material due to the natural settling effects of the limeparticles. If particles of this type are permitted to settle naturally,it will be found that a `Self-classifying effect occurs inl which thefiner particles segregate toward the center of the cone, and the coarserparticles tend to segregate toward the edges. Since the smaller, moreclosely packed particles in the center of the cone present the greatestresistance to air flow, attempts to cool this mass by passing airupwardly through it frequently result in channeling the air toward theedges of the mass, leaving the center portionl substantially uncooled.Coupled with the greaterresistance tov air flow through the centralportion is what is known as the wall effect arising from the fact thatthe particles located directly along the walls of the chamber are not asclosely interpackcd as the rest of the bed. Consequently, a smaller llowresistance occurs along the wal-l, and higher air llows occur at thepath of least resistance presented at the boundary between the particlesand the wall. The cumulative result of all these factors is rapidcooling of the particles toward the edge of the built-up cone, andinsutlicient cooling at the center where the liner particles arelocated.

To minimize the eifects of this type of distribution, the' particlesleaving the Soaking zo'ne 16 are transferred into a plurality ofindividual streams into a cooling zone generally indicated at numeral 17in the drawings. The discharge means employed for discharging particlesfrom the soaking zone 16 into the cooling zone 17 may take the form of aplurality of hoppers 19 each consisting of a frusto-pyrimidical sectionterminating at its smaller end in a restricted throat 21.

The particles entering the cooling zone 17 therefore are separated intofour distinct streams substantially equal in area. Somevself-classifying may occur within the cooling zone 17 itself, resultingin the production of conically shaped peaks 22 on the bed of materialsettling in the cooling zone 17, but the problems of heat transfer willnot nearly be as great as if the mass were allowed to settle into asingle conically shaped bed. It will be found that the fines passingfrom the soaking zone 16 will be more or less equally distributed ineach of the four streams by entering the cooling zone 17.

To provide the cooling effect desired, the cooling zone 17 is providedwith a plurality of air inlet means which may include a centrallydisposed air inlet 23 in combination with a plurality of secondary airinlet means 24 at spaced intervals about the central conduit 23. Formost efllcient cooling, the secondary air inlet means Zd should belocated in line with each of the hoppers 19 as they discharge theirstreams of material into the cooling zone 17. A baille 25 is providedover the central conduit 23, and bafiles 26 are provided over thesecondary air inlets in order to diffuse the air as it enters thecooling zone and to prevent accumulation of the particles within theconduits themselves. The introduction of air centrally of the dischargezone also is effective in preventing a down-draw of material through thecenter of the hopper.

The air supply for the cooling zone is received through l one or moreair inlets 27 located below the cooling zone proper, and fed from asource of pressured air. A plurality ofy b'aflled hoppers 28 receive theparticles falling by gravity from the bed in the cooling zone andtransfer them to suitable feeding or other handling equipment.

In order to permit, efficient soaking of the particles in the soakingzone 16, without the addition of extraneous heat, most of the airpassing through the cooling zone 17 should be by-passed about thesoaking zone 16. For this purpose, the throats 21 are madewith'relatively small cross-sections and of extended length so thatthere is a high resistance flow path to the soaking zone, and thecooling and soaking zones are provided with an end wall 29 which isspaced a substantial distance from the outer wall 3% of the unit. Thespace between the walls 29 and 30, identified at numeral 31 in thedrawings, thereby provides a by-pass duct of substantial area forhandling the large quantities of air which must be ellectively removedfrom the cooling zone. This construction also eliminates the possibilityof dust accumulation in the spaces adjoining the soaking and coolingzones. The large open area at the surface of the bed also insures lowair exit velocities from the bed and thus minimizes dust pickup.

As previously explained, it is desirable to increase the efliciency ofthe calcining operation in the soaking zone 16 by passing some of thecooling air through the soaking zone as a purging gas. For this purpose,a plurality of baflles 32 are disposed between the cooling zone 17 andthe duct 31 to regulate and proportion the amount of air being sent intothe duct 31 and that being sent into the soaking Zone 16. The correctproportion of air to be by-passed can be readily determined throughexperience. The carbon dioxide content in the soaking zone and thepressure at the top' of the cooling zone will be the determining factorsin making this adjustment. An adjustable baille 36 is provided toprevent the tangential current of air from creating a swirling action inthe flame firing the kiln 10.

Generally, the by-pass duct 31 must handle about of the total cooler airso that the air being used to purge the soaking zone is about 5% of thatgoing through the cooling zone. A duct of the type described which has alarge open area at the top of the cooling zone, and a large, easilycleaned duct can ellectively bypass sufficient air to provide thedesired cooling and purging effect. Attempts to accomplish the sameeffect with pipes or other conduits extending between the cooling zoneand the soaking zone have not been successful because of excessive dustpick-up and excessive flow resistance, so that the proper proportioningof the cooling air has not been achieved.

A modified form of the present invention is illustrated in Figure 4 ofthe drawings. In this form of the invention, a duct 33 is providedbetween a forward wall 3f!- and an outer wall 35, the duct 33 being ofsufficient cross-sectional area to handle the required volume of air.The advantage of this arrangement is the fact that the air passes to thehood and kiln in a direct axial path, rather than in a tangential path,so that the llame of the kiln is less likely to be affected by the airstream.

The apparatus of the present invention provides an efcient buteconomical means for controlling the distribution of air between acombined soaking and cooling zone. The cooling takes place withoutsubstantial interference with the calcining operation, and the sensibleheat in the air passing through the cooling zone can be readilyrecovered. Likewise, the soaking operation is carried out withoutinterference from high velocity air streams, but with sutlicient airbeing present to move the calcining reaction toward completion, toprovide a lower carbon dioxide content in the soaking zone, to increasethe heat transfer rate, and to increase the total heat available forcalcinat'ion.

It will be evident that various modifications can be made to thedescribed embodiments without departing from the scope of the presentinvention.

I claim as my invention:

l. An apparatus for heat treating calcareous particles comprising spacedwalls defining a soaking zone and a cooling zone, a plurality of angularhoppers disposed between said soaking zone and said cooling zone, saidhoppers having reduced throat portions arranged to confine the flow ofparticles from said soaking zone into said cooling zone into separatestreams providing relatively high resistance paths for air flow fromsaid cooling zone into said soaking zone, means for introducing air intosaid cooling zone, and wall means spaced from said spaced wallsproviding with said spaced walls a passageway in open fluidcommunication with said cooling zone having a relatively low resistanceto air flow from said cooling zone.

2. The apparatus of claim l including damper means 5 arranged toregulate the amount of air passing into said References Cited in thefile of this patent passageway.

3. The apparatus of claim 1 in which said passageway UNITED STATESPATENTS is substantially coextensive in length with the combined2,137,158 Douglass Nov. 15, 1938 length of said soaking and coolingzones. 5 2,653,809 Azbe Sept. 29, 1953

